1. Field of the Invention
The present invention relates to an illumination optical system that converts non-polarized light rays emitted from a light source into one type of linearly polarized light beams having a substantially fixed direction of polarization. The present invention also pertains to a projector that utilizes the illumination optical system to display bright images.
2. Description of the Related Art
The projector modulates illumination light transmitted to an electro-optical device called xe2x80x98light valvexe2x80x99 according to image information regarding an image to be displayed and projects the modulated light on a screen to display the resulting image. A liquid crystal light valve utilizing the functions of a liquid crystal panel is generally used for the electro-optical device.
It is naturally desirable to display bright images by the projector. For this purpose, the illumination light emitted from an illumination optical system incorporated in the projector preferably has the high utilization efficiency. The liquid crystal light valve included in the projector generally utilizes only one type of linearly polarized light beams. In the case of an illumination optical system that emits non-polarized light beams, light beams having a certain direction of polarization are not utilized in the liquid crystal light valve. This worsens the utilization efficiency of the illumination light emitted from the illumination optical system. For the enhanced utilization efficiency of the illumination light by the liquid crystal light valve, some proposed illumination optical systems use a polarization conversion optical system that converts the non-polarized light beams emitted from a light source into one type of linearly polarized light beams.
FIG. 13 is a plan view schematically illustrating the structure of a main part of a prior art illumination optical system. In the description below, the direction of advance of light is defined as a z-axis direction (direction in parallel with a system optical axis SX of the illumination optical system), the direction of 12 o""clock relative to the direction of advance of light is defined as a y-axis direction (vertical direction), and the direction of 3 o""clock relative to the direction of advance of light is defined as an x-axis direction (horizontal direction). The illumination optical system includes a light source 1110, a first lens array 1120, a second lens array 1130, a polarization conversion optical system 1140, and a superimposing lens 1150, which are arranged sequentially along the system optical axis SX. The first lens array 1120 has a plurality of minute lenses 1122. The second lens array 1130 has a plurality of minute lenses 1132, which respectively correspond to the plurality of minute lenses 1122 of the first lens array 1120.
The polarization conversion optical system 1140 includes plural pairs of mutually parallel polarizing separation films 1142 and reflection films 1144, which are arranged sequentially in the x-axis direction. The polarizing separation films 1142 and the reflection films 1144 are inclined to have a fixed angle relative to the z-axis direction. xcex/2 phase plates 1148 are attached to the outgoing faces of the respective polarizing separation films 1142.
Substantially parallel light rays emitted from the light source 1110 are divided into a plurality of sub-beams by the plurality of minute lenses 1122 of the first lens array 1120. The condensing functions of the respective minute lenses 1122 of the first lens array 1120 and the minute lenses 1132 of the second lens array 1130 cause the plurality of sub-beams to be condensed in the vicinity of the polarizing separation films 1142 in the polarization conversion optical system 1140. An optical axis LX of the light source 1110 is displaced in a xe2x88x92x-axis direction from the system optical axis SX of the illumination optical system by a predetermined amount of displacement Dp (=Wp/2), in order to allow the plurality of sub-beams emitted from the first lens array 1120 to efficiently enter the polarizing separation films 1142 of the polarization conversion optical system 1140. Here Wp denotes the distance between the polarizing separation film 1142 and the reflection film 1144.
Out of the light beams condensed in the vicinity of the polarizing separation films 1142, one type of linearly polarized light beams (for example, p-polarized light beams) are mostly transmitted by the polarizing separation films 1142, whereas the other type of linearly polarized light beams (for example, s-polarized light beams) are mostly reflected by the polarizing separation films 1142. The other type of linearly polarized light beams reflected by the polarizing separation films 1142 are further reflected by the reflection films 1144 and enter the superimposing lens 1150. One type of linearly polarized light beams transmitted by the polarizing separation films 1142, on the other hand, enter the xcex/2 phase plates 1148 to be converted to linearly polarized light beams having the same direction of polarization as that of the other type of linearly polarized light beams, and subsequently enter the superimposing lens 1150. The plurality of sub-beams entering the superimposing lens 1150 are substantially superimposed on an area LA to be illuminated. The prior art illumination optical system thus enables the area to be illuminated with substantially one type of linearly polarized light beams.
FIG. 14 shows drawbacks of the prior art illumination optical system. FIG. 14A shows the optical path of the light transmitted by the polarizing separation film 1142 (hereinafter may be simply referred to as the xe2x80x98transmitted lightxe2x80x99). FIG. 14B shows, on the other hand, the optical path of the light reflected by the polarizing separation film 1142 and the reflection film 1144 (hereinafter may be simply referred to as the xe2x80x98reflected lightxe2x80x99). For the simplicity of explanation, in the illustration of FIGS. 14A and 14B, the optical path of the reflected light by the polarizing separation film 1142 and the reflection film 1144 is replaced by an equivalent linear optical path, and deflection of light by the superimposing lens 1150 is neglected.
As shown in FIGS. 14A and 14B, in the prior art illumination optical system, out of the light beams entering the polarizing separation film 1142, an optical path length of the reflected light between the polarizing separation film 1142 and the area LA is greater than an optical path length L2 of the transmitted light by the distance Wp between the polarizing separation film 1142 and the reflection film 1144. The difference in optical path length makes the size of a second region W2 on the area LA, which is illuminated with the reflected light, greater than the size of a first region W1 on the area LA, which is illuminated with the transmitted light. This causes the illuminating efficiency with the reflected light to be lower than the illuminating efficiency with the transmitted light, thus significantly reducing the total illuminating efficiency in the illumination optical system.
The object of the present invention is thus to provide a technique that prevents the significant reduction of the illuminating efficiency occurring in the prior art illumination optical system that utilizes the polarization conversion optical system.
At least part of the above and the other related objects is attained by an illumination optical system that emits illumination light. The illumination optical system includes: a light source that emits non-polarized light; a polarization conversion optical system that has at least one pair of a polarizing separation film and a reflection film, which are arranged to be mutually in parallel with each other and to be inclined relative to a predetermined direction, the polarization conversion optical system converting incident non-polarized light into linearly polarized light having a predetermined direction of polarization; a first optical system that has at least one first lens, which is arranged on an optical path between the light source and the polarizing separation film to emit a first light beam; and a second optical system that has a second lens, which receives an incident second light beam that is a component of the first light beam emitted from the first lens and is transmitted by the polarizing separation film, and a third lens, which receives an incident third light beam that is another component of the first light beam emitted from the first lens and is reflected by the polarizing separation film and the reflection film. Here an optical property of at least one of the second lens and the third lens is adjusted to make a size of a first region on a predetermined area, which is irradiated with the second light beam, substantially equal to a size of a second region on the predetermined area, which is irradiated with the third light beam.
In the illumination optical system of the present invention, the adjustment is carried out to make the size of the first region on the predetermined area, which is irradiated with the second light beam, substantially equal to the size of the second region on the predetermined area, which is irradiated with the third light beam. This arrangement effectively prevents the significant reduction of the illuminating efficiency occurring in the prior art illumination optical system that utilizes the polarization conversion optical system. The technique of the present invention thus actualizes the illumination optical system having the high illuminating efficiency.
In accordance with one preferable application of the illumination optical system of the present invention, the adjustment displaces the position of a curved face of the third lens from the position of a curved face of the second lens.
The adjustment of displacing the position of the curved face of the third lens from the position of the curved face of the third lens makes the size of the first region on the predetermined area, which is irradiated with the second light beam, substantially equal to the size of the second region on the predetermined area, which is irradiated with the third light beam.
Here it is preferable that the second lens and the third lens have curved faces of an identical shape.
The simple adjustment of displacing the position of the curved face of the third lens from the position of the curved face of the second lens thus readily makes the size of the first region on the predetermined area, which is irradiated with the second light beam, substantially equal to the size of the second region on the predetermined area, which is irradiated with the third light beam.
In the illumination optical system of the present invention, a difference Di between the position of the curved face of the second lens and the position of the curved face of the third lens is determined by an equation given below:
Di=(Wpxc2x7Wi)/(Wi+W)
where Wi denotes a size of the first lens, W denotes the size of the first area, and Wp denotes a distance between the polarizing separation film and the reflection film.
This application readily determines the relationship between the position of the curved face of the second lens and the position of the curved face of the third lens.
The present invention is also directed to a projector that projects an image. The projector includes: an illumination optical system that emits illumination light; an electro-optical device that modulates the light emitted from the illumination optical system according to image information; and a projection optical system that projects the modulated light by the electro-optical device. The illumination optical system includes: a light source that emits non-polarized light; a polarization conversion optical system that has at least one pair of a polarizing separation film and a reflection film, which are arranged to be mutually in parallel with each other and to be inclined relative to a predetermined direction, the polarization conversion optical system converting incident non-polarized light into linearly polarized light having a predetermined direction of polarization; a first optical system that has at least one first lens, which is arranged on an optical path between the light source and the polarizing separation film to emit a first light beam; and a second optical system that has a second lens, which receives an incident second light beam that is a component of the first light beam emitted from the first lens and is transmitted by the polarizing separation film, and a third lens, which receives an incident third light beam that is another component of the first light beam emitted from the first lens and is reflected by the polarizing separation film and the reflection film. Here an optical property of at least one of the second lens and the third lens is adjusted to make a size of a first region on the electro-optical device, which is irradiated with the second light beam, substantially equal to a size of a second region on the electro-optical device, which is irradiated with the third light beam.
The projector of the present invention utilizes the illumination optical system of the present invention discussed above, thus enabling the electro-optical device to be irradiated with light at a higher efficiency, compared with a projector that utilizes the prior art illumination optical system. The projector of the present invention thus enables brighter images to be displayed.